Originally Posted by
Lille-bror
Gentlemen
How does your glowplugs looks, after some liters ( gallon) of fuel?
My experience:
With a well tuned engine (not too lean or rich), the color on the coil will be like it was new - even after many liters of fuel.
The coil will be grey or black if you your engine is set too lean or if you use too cold plug for the specific engine/fuel/compression ratio.
Hit me back
When my tune is where I want it my plugs have a nice cocoa brown coloring around the base of the plug and the filiment looks new. When you see white or grey for a color it is lean and the filiment may be distorted. If you see really dark brown or black, it is rich. I have also seen black on the top of the piston if the oil package in the fuel isn't as good of a quality or if the engine has run hot for a period of time. There are other reasons for certain plug colors but these are the basics.